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2026-10

Chemical‑Etching Manufacturing Process Deep‑Dive for 0.4 mm‑Thick Letter‑Tiles of Seed‑Phrase Capsule

Introduction

Letter‑tile is the functional core of stacked‑tile seed‑phrase capsule, nominal thickness 0.4 mm 304 stainless‑steel. Every tile contains central mounting‑slot and etched character marking. These ultra‑thin parts are manufactured via chemical‑etching. Comparing with ordinary thick‑sheet etching, 0.4 mm thin‑plate faces unique process challenges: photoresist‑coating uniformity, pattern‑alignment accuracy, isotropic‑etch undercut management, post‑etch warpage induced by residual‑stress, thorough rinsing‑cleaning to eliminate etchant residue. This article dissects key production‑process stages.

1. Raw‑Material Preparation & Pre‑Treatment

Raw‑material: stress‑relieved AISI 304 stainless‑steel sheet, target thickness 0.4 mm, thickness tolerance ±0.03 mm.

Critical: select stress‑relieved annealed material. Non‑annealed cold‑rolled thin‑sheet carries high internal residual‑stress; after etching removing surrounding material, stress‑release will make individual letter‑tiles severely warp.
Pre‑treatment steps: degreasing, alkaline‑cleaning, surface activation. Oil stain and surface contamination will cause photoresist delamination in subsequent processes, generating tile‑edge defect and character‑pattern missing.

2. Photoresist Coating & Pattern Exposure

For 0.4 mm ultra‑thin sheet, photoresist thickness uniformity directly determines character‑etching quality and slot‑edge quality.

Double‑side photoresist lamination / spin‑coating. Coating‑thickness fluctuation must be tightly controlled. Uneven resist‑thickness creates inconsistent etch‑resistance, leading to variable undercut dimension.
Artwork pattern: includes alphabet‑character graphic, central‑slot contour, sheet‑level micro‑bridge connections (to keep tiles attached to mother‑sheet during processing). Micro‑bridge width needs DFM optimisation: too‑narrow bridge fractures mid‑process; too‑wide bridge creates heavy burr‑risk at break‑off location.
Double‑sided pattern‑alignment: letter‑tile is through‑etched part; front‑back pattern mis‑alignment will deform central‑slot geometry and character‑outline. Precision exposure‑alignment system is required.

3. Isotropic Chemical‑Etching & Etch‑Factor Compensation

Chemical‑etching is isotropic: material removes vertically and laterally simultaneously. Lateral undercut magnitude is described by etch‑factor. For 0.4 mm 304 sheet, lateral undercut will modify central‑slot opening and character‑feature dimension.

Artwork must apply pre‑compensation offset according to real‑measured etch‑factor from process trial runs. CAD drawing dimension cannot be directly copied onto phototool.
Process variables that shift etch‑factor: etchant concentration, fluid spray‑pressure, bath‑temperature, etching dwell‑time, material grain‑structure.
Over‑etching risk: excessive dwell‑time enlarges slot width beyond tolerance, erodes character‑profile definition. Under‑etching causes incomplete through‑cut, tiles remain connected by thin metal web.

4. Photoresist Stripping, Sheet De‑Bonding & Tile Separation

After etching completion:

Chemical stripping removes photoresist layer from both‑sides of sheet. Incomplete resist‑stripping leaves polymer residue on tile‑surface, which may trap chemical contaminant.
Letter‑tiles are still held on mother‑sheet via pre‑designed micro‑bridges. Operators separate individual tiles by breaking micro‑bridges.

Process risk point: Micro‑bridge fracture‑points may generate micro‑burr. Post‑separation visual inspection is mandatory for every batch.

5. Post‑Etch Cleaning & Passivation (Critical for Long‑Term Corrosion‑Resistance)

Residual ferric‑chloride‑based etchant trapped in micro‑crevices of letter‑tile is a hidden failure source: leftover etchant slowly triggers pitting‑corrosion on 304 stainless‑steel during multi‑year storage. Process flow: multi‑stage cascade water‑rinsing → neutralisation treatment → passivation → final pure‑water rinse → drying. Passivation rebuilds chromium‑oxide protective film on stainless‑steel surface, improving long‑term anti‑corrosion performance for cold‑storage backup component.

6. Flatness Correction & Quality Inspection

0.4 mm thin tiles still may exhibit minor warpage after whole process, even using stress‑relieved raw‑stock.

Implement flat‑calibration for warped tiles. Target flatness specification ≤ 0.05 mm. Warped tiles will damage overlapping‑stack effect inside capsule and degrade X‑ray‑obscuring performance.
Batch inspection checklist:

Tile thickness dimension
Central‑slot width tolerance
Character‑etch definition & completeness
Edge burr / micro‑fragment inspection under tool‑microscope
Tile flatness measurement
Surface‑residue & contamination visual‑check

7. Main Mass‑Production Yield‑Limiting Bottlenecks

Post‑etch tile warpage from residual‑stress release (most dominant yield‑loss source for ultra‑thin etched parts).
Micro‑burr formed at micro‑bridge break‑off location.
Character‑pattern distortion from double‑side exposure mis‑registration.
Residual‑etchant contamination caused by insufficient multi‑stage rinsing.

Process‑engineer should tune raw‑material annealing‑status, micro‑bridge DFM parameters, etching‑time window and cleaning workflow to lift overall final yield rate.

Conclusion

0.4  mm letter‑tile is seemingly simple alphabet‑tag part, but ultra‑thin‑sheet chemical‑etching chain contains multiple hidden bottlenecks. Raw‑material stress‑relief, photoresist‑coating quality, etch‑factor compensation, thorough post‑etch cleaning and flat‑calibration collectively decide final‑product quality and long‑term storage reliability of seed‑phrase capsule.

About NKEYTO NKEYTO is the international brand of Shenzhen Xintu Precision Hardware Co., Ltd., a precision metal manufacturing supplier based in Shenzhen, China. We provide one‑stop precision metal manufacturing services, including chemical etching, precision laser cutting, stamping, CNC machining, electroplating, and other customized surface treatments. Our capabilities support the development and production of precision metal components for industries such as semiconductors, electronics, medical devices, and industrial applications. With engineering support and flexible manufacturing capabilities, we help customers develop customized metal parts from prototypes to small and medium‑volume production. For technical evaluation or quotation, please feel free to contact: Luna Head of International Trade & Pre‑Sales Technical Consultant Email: luna@nkeyto.com WhatsApp: +1 213 221 9094 Phone / WeChat: +86 135 5470 8126